This invention relates to a dispersion of a functional silicone modifier in a curable epoxy resin wherein emulsification of said silicone is aided by a siloxane-polyoxyalkylene copolymer. More specifically, the present invention relates to a dispersion of an alpha,omega-functional polydimethylsiloxane having carboxyl, amine or epoxide endgroups. This invention further relates to a method of using the dispersion to effect improved impact resistance of the resin when the latter is cured. Still further, this invention relates to an article of manufacture which comprises said dispersion in a cured state.
Epoxy resins form an important class of thermosetting polymers that generally exhibit high tensile strength, high modulus, good chemical and corrosion resistance and good dimensional stability. Unfortunately, cured epoxy systems are usually brittle, which characteristic severely limits their utility in many applications requiring high impact and fracture strength. This shortcoming has been overcome to some extent by the well-established technique of incorporating rubber particles into the cured epoxy matrix. The rubber tends to separate into a second dispersed phase during the cure of the epoxy resin. It is believed that these rubbery domains act to relieve stress during deformation, which results in improved toughness and impact resistance of the so-modified epoxy resin. Thus, for example, liquid butadiene-acrylonitrile copolymers, having either carboxyl or amine end groups, have been widely used to modify epoxy resins.
Even though modification with relatively small quantities of such materials can lead to significant improvement in impact resistance, further augmentation is still desirable. Moreover, these copolymeric modifiers themselves suffer from characteristics which limit their utility to a narrow temperature range, namely a relatively high glass transition temperature and poor thermal and oxidative stability. Silicones, on the other hand, are known to have good thermal stability as well as applicability over a wide temperature range. Silicones have indeed been disclosed as impact resistance modifiers for epoxy resins, but it is only certain expensive varieties that have demonstrated utility for this purpose. Japanese Patent No. 60/049,023 discloses organo-polysiloxanes containing perfluoroalkyl groups which, in combination with an inorganic filler, may be used to improve impact resistance of an epoxy resin. Likewise, Yorkgitis et al. ("Siloxane Modifiers for Epoxy Resins," Govt. Accession No. AD-A136,782, Dec. 1, 1983), found that modification with amine-terminated copolymers of dimethylsiloxane with diphenylsiloxane or with methyltrifluoropropylsiloxane led to improved fracture toughness in a bisphenol-A epoxy resin while a similarly terminated homopolymer of polydimethylsiloxane actually reduced toughness relative to the control. It would, therefore, be advantageous if the less expensive and readily available polydimethylsiloxanes could be used to improve the impact resistance of epoxy resins.
It is known that non-organofunctional polydimethylsiloxane fluids and gums can be dispersed in various resins with the aid of dispersants based on siloxane resin-polyoxyalkylene copolymers or polydimethylsiloxane-polyoxyalkylene copolymers. Keil, in U.S. Pat. Nos. 3,843,577 and 3,926,885, the disclosures of which are hereby incorporated by reference, described such dispersions of non-organofunctional polydiorganosiloxanes in epoxy resins in order to effect a self-lubricating surface thereon. Compositions taught by Keil, however, exhibited little or no improvement of impact resistance over unmodified controls.
Unexpectedly, it has now been found that when a polydimethylsiloxane fluid having carboxyl, amine or epoxide terminal groups is dispersed in a curable epoxy resin, significant improvement in impact resistance of the cured system results.